How to Choose Between Molded and Pultruded FRP Grating
Two manufacturing processes produce the two families of FRP grating most specified worldwide: molded and pultruded. Both outperform steel in corrosive environments, but they serve different structural roles.
Molded Grating — Bi-Directional Strength
Compression molding interlocks continuous glass fibers in two directions, giving equal strength in both panel axes. Impact resistance is excellent, which is why molded grating dominates chemical plants, wastewater facilities and areas with dropped tools and vehicle traffic.
Pultruded Grating — Twice the Stiffness
Pultrusion packs up to 70% glass content into aligned bearing bars, roughly doubling stiffness versus molded panels. Longer spans, heavier loads and less deflection make pultruded the choice for high-traffic walkways, mezzanines and offshore decks.
The Decision Framework
Choose molded when: impact resistance matters, loads are moderate, spans under ~1.2 m, or chemical exposure requires resin flexibility. Choose pultruded when: spans exceed 1.5 m, loads are heavy, deflection is critical, or maximum stiffness-to-weight is required. Many projects use both — molded for process areas, pultruded for main walkways.
ZeAllgrate manufactures both, so recommendations are based on your application, not our inventory.
Structural Differences That Drive Selection
The two processes produce genuinely different materials. Compression molding interlaces chopped and continuous glass in two directions, delivering balanced strength in both panel axes — with typical glass content of 30–35%. Pultrusion pulls continuous roving through a heated die, packing up to 65–70% glass into aligned bearing bars, roughly doubling stiffness along the bearing direction. That single difference drives almost every selection decision: stiffness, span, deflection and the directionality of load capacity.
| Characteristic | Molded Grating | Pultruded Grating |
|---|---|---|
| Glass content | 30–35% | 65–70% (bearing bars) |
| Strength orientation | Bi-directional (equal both axes) | Strong along bearing direction |
| Relative stiffness | Baseline | ≈ 2× molded |
| Typical clear span | Up to ~1.2 m | 1.5–3 m |
| Impact resistance | Excellent (continuous interlock) | Good (unidirectional bars) |
| Open area options | Wide range (incl. micro-mesh) | Standard mesh patterns |
| Best first cost | Lower | Higher (more glass, denser bars) |
Load and Deflection Benchmarks
At equal thickness and equal span, a pultruded panel will show roughly half the deflection of a molded panel at the same load — the direct consequence of the higher modulus. That is why traffic decks, mezzanines and offshore access routes with long spans specify pultruded. Molded panels are not weaker in any absolute sense: they are specified where bi-directional strength, impact and mesh options matter more than span — process areas, chemical plant floors, drainage covers and trench covers where forklift and dropped-load impact is the governing load case.
Decision Matrix
- Choose molded when: spans are under ~1.2 m, loads are moderate, impact resistance matters, you need fine mesh or high open area, or chemical flexibility across many exposure types is the priority.
- Choose pultruded when: spans exceed 1.5 m, loads are heavy, deflection is a code limit, or maximum stiffness-to-weight is required — offshore decks, vehicle-capable walkways, stair treads and mezzanines.
- Use both on large plants: molded in process areas where chemical splash and impact dominate, pultruded for main corridors and access walkways where span governs. This is the standard layout on chemical and offshore projects — and because ZeAllgrate manufactures both, the recommendation follows your application, not our inventory.
Common Misconceptions
- "Pultruded is always better." Not for impact: molded's continuous interlock absorbs dropped tools and vehicle loading better than the unidirectional bars of pultruded grating.
- "Molded is only for light duty." Molded panels with adequate thickness carry forklift traffic in process plants; the limit is span, not duty.
- "One family fits the whole plant." Large facilities almost always mix both — the economics and performance of each improve when each is placed where it genuinely wins.
Mixed-Construction Practice
The molded-vs-pultruded decision is rarely site-wide: long access spans resolve to pultruded, while platforms, equipment zones and impact areas often stay molded. The two constructions combine cleanly because both carry the same resin options, surface finishes and clip hardware — a deck can be pultruded on the runways and molded on the working platforms without an interface problem. Order both on one drawing set so spans, orientation and clip patterns are coordinated, and note that a mixed package lets the engineer optimise cost and stiffness zone by zone instead of choosing one construction for the whole plant.
The molded-vs-pultruded decision is rarely site-wide: long access spans resolve to pultruded, while platforms, equipment zones and impact areas often stay molded. The two constructions combine cleanly because both carry the same resin options, surface finishes and clip hardware — a deck can be pultruded on the runways and molded on the working platforms without an interface problem. Order both on one drawing set so spans, orientation and clip patterns are coordinated, and note that a mixed package lets the engineer optimise cost and stiffness zone by zone instead of choosing one construction for the whole plant.Discuss Your FRP Project
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